1. Field of the Invention
The present invention relates to the field of catalysts for hydrogenation of carbon dioxide. In particular, the present invention relates to supported catalysts for hydrogenation of carbon dioxide wherein the catalyst support is coated with a material capable of catalyzing a reverse water-gas shift reaction.
2. Description of the Related Technology
Increasing awareness of the environmental impact of carbon dioxide (CO2) emissions has lead to an immense increase in research and development efforts to bind CO2. Proposals range from capturing CO2 directly from the flue gas emitted by heavy industry or from the atmosphere by binding it in inorganic oxides. Avalos-Rendon, et al., Journal of Physical Chemistry A 113, 6919 (2009) and Nikulshina, V., et al., Chemical Engineering Journal 146 (2), 244 (2009). One approach is to reduce the CO2 over catalysts, to convert it to more valuable hydrocarbons using photochemical, electrochemical or thermochemical processes.
Electrochemical and photochemical CO2 conversion is still in its infancy and at present has major drawbacks. Photocatalysts tend to require a sacrificial electron donor. Collin, J. P. and Sauvage, J. P., Coordination Chemistry Reviews 93 (2), 245 (1989) and Fujita, E., Hayashi, Y., Kita, S., and Brunschwig, B. S., Carbon Dioxide Utilization for Global Sustainability 153, 271 Sustainability 153, 271 (2004). Further, neither photocatalytic nor electrocatalytic conversion of CO2 yield long chain hydrocarbons nor do they show very high CO2 conversion efficiencies. Noda, H. et al., Bulletin of the Chemical Society of Japan 63 (9), 2459 (1990).
Thermochemical CO2 conversion, in contrast, has been known for several decades and is presently the most proven and successful approach to producing hydrocarbons (HC) above methane at high conversion yields. Russell, W. W. and Miller, G. H., Journal of the American Chemical Society 72 (6), 2446 (1950) and Dorner, R. W., Hardy, D. R., Williams, F. W., and Willauer, H. D., Applied Catalysis A: General (2009). This research is primarily driven by the U.S. military's significant demand for jet fuel and the associated target of increasing energy independence and battlefield readiness as well as reducing CO2 emissions, in light of the impending introduction of the cap-and-trade system. One can envisage a process leading to jet fuel, where the needed carbon source is obtained by harvesting CO2 dissolved in the ocean (primarily in the form of bicarbonate) and hydrogen through the electrolysis of water. Willauer, H. D.; et al., Energy & Fuel 23, 1770 (2009) and Willauer, H. D., et al., “Effects of Pressure on the Recovery of CO2 by phase Transition from a Seawater System by Means of Multilayer Gas Permeable Membranes”, J Phys Chem A, in press (2009). CO2 and H2 can subsequently be reacted over a heterogeneous catalyst to form hydrocarbons of desired chain length and type.
A key problem with this scenario is the low conversion yield of CO2 hydrogenation processes. Thus, a significant increase in the conversion yield of CO2 hydrogenation catalysts will enhance the feasibility of the above-mentioned process.
The target of achieving a high yield, high selectivity process for CO2 hydrogenation to jet fuel can be achieved by use of a two step synthesis process, involving initial CO2/H2 conversion to olefins and subsequent oligomerization over a solid acid catalyst to jet fuel. Even when using syngas (CO/H2), direct synthesis of jet fuel is limited by Anderson-Schulz-Flory (ASF) kinetics to a selectivity of around 50%. However, this type of selectivity can only be achieved when employing a catalyst that exhibits an extremely high chain growth probability of 0.9, which in CO2 hydrogenation has not been observed before. Van der Laan, G. P. and Beenackers, A., Catalysis Reviews—Science and Engineering 41 (3-4), 255 (1999). Consequently, a two-step process is advantageous in comparison to the direct route to jet-fuel.
A conversion of 41.4% of CO2/H2 over a K/Mn/Fe catalyst supported on alumina and an olefin/paraffin ratio of 4.2 has been reported. Dorner, R. W., Hardy, D. R., Williams, F. W., and Willauer, H. D., Applied Catalysis A: General (2009). Initial tests on a cobalt-based catalyst yielded predominantly methane (CH4), with no carbon monoxide (CO) detected in the product effluent. Dorner, R. W. et al., Energy Fuels 23 (8), 4190 (2009).
The conversion of CO2 to long chain hydrocarbons has been established to go through a 2-stage reaction mechanism over iron catalysts, with initial conversion of CO2 to CO on the iron's magnetite phase (Lox, E. S, and Froment, G. F., Industrial & Engineering Chemistry Research 32 (1), 71 (1993)), followed by chain growth as observed in Fischer-Tropsch (FT) synthesis on iron carbide surface species. Riedel, T., et al., Industrial & Engineering Chemistry Research 40 (5), 1355 (2001); Bukur, D. B., et al., Journal of Catalysis 155 (2), 366 (1995); Herranz, T. et al., Applied Catalysis a—General 311, 66 (2006); and Li, S. Z. et al., Journal of Catalysis 206 (2), 202 (2002).
In cobalt-systems however, the predominant reaction seems to be CO2 conversion directly to methane due to cobalt's limited water-gas shift (WGS) activity. Based on this model, the approach within entails the development of a bifunctional catalyst that includes both reverse water-gas shift (RWGS) activity as well as FT chain growth activity on the catalyst's surface. The addition of a second, separate reverse water gas shift (RWGS) catalyst to a cobalt Fischer-Tropsch catalyst within the same reactor would not suffice in achieving CO2 conversion to long chain HC, as due to thermodynamic restrictions the carbon monoxide's partial pressure within the reactor would remain too low and insufficient to establish an FT regime. Riedel, T. et al., Applied Catalysis A—General 186 (1-2), 201 (1999).
It is known, that the RWGS reaction can take place over promoted ceria at modest temperatures, with an equilibrium constant of 16% reported over a Pd/CeO2 catalyst at 300° C. and an equimolar CO2: H2 feed. Pettigrew, D. J., Trimm, D. L., and Cant, N. W., Catalysis Letters 28 (2-4), 313 (1994). However, if one replaced palladium with iron, a lower equilibrium constant can be expected, as iron catalyses the RWGS to a lesser extent than palladium does. Hilaire, S. et al., Applied Catalysis A—General 258 (2), 269 (2004). The RWGS takes advantage of ceria's oxygen storage ability, involving the redox process over the Ce4+/Ce3+ couple. It has been proposed that the reaction proceeds via reduction of CeO2 by hydrogen to Ce2O3, producing water in the process. Subsequently CO2 can then be expected to re-oxidize Ce2O3, restoring the initial CeO2 species and yielding CO. Pettigrew, D. J., Trimm, D. L., and Cant, N. W., Catalysis Letters 28 (2-4), 313 (1994). Rates are however partially limited by H2O re-oxidizing Ce2O3. Hilaire, S. et al., Applied Catalysis A—General 258 (2), 269 (2004). The addition of base metals to ceria is known to be beneficial for the RWGS, by reducing the activation energy and increasing the reducibility of ceria. Li, K., Fu, Q., and Flytzani-Slephanopoulos, M., Applied Catalysis B-Environmental 27 (3), 179 (2000).
In WO 96/06064 A1 a process for methanol production is described, which comprises a step of converting part of the carbon dioxide contained in a feed mixture with hydrogen to carbon monoxide, in the presence of a catalyst that can be used for the WGS reaction; exemplified by Zn—Cr/alumina and MoO3/alumina.
WO 2005/026093 A1 discloses a process for producing DME, which comprises a step of reacting carbon dioxide with hydrogen in a RWGS reactor to provide carbon monoxide, in the presence of a supported catalyst selected from ZnO; MnOx (x=1˜2); an alkaline earth metal oxide and NiO.
EP 1445232 A2 discloses a (reverse) water gas shift reaction for production of carbon monoxide by hydrogenation of carbon dioxide at high temperatures, in the presence of a Mn—Zr oxide catalyst.
A drawback of the known process as disclosed in US 2003/0113244 A1 is the selectivity of the catalyst employed; that is no long chain hydrocarbons are formed. Energy intense conversion of CO2 to CO has to occur prior to upgrading, in a separate reactor.
The object of the present invention is therefore to provide a catalyst that shows improved selectivity and yield in reducing carbon dioxide with hydrogen, with only very little methane formation, and with good catalyst stability.